This invention relates to a light emitting diode excellent in larger quantity production and, in particular, to a light emitting diode of a double heterojunction structure using a compound semiconductor material.
Recently, optical subscriber transmission systems are beginning to acquire a quick and wide acceptance in the art. Among them, a light emitting diode (LED) can be manufactured at low cost with high reliability as compared with a laser diode (LD) and does not encounter, for example, any modal noise resulting from a combination of the LD with multi-mode fiber. From this, it will be understood that the LED is a key device of the above-mentioned optical subscriber transmission system.
Though inexpensive, the LED is very much higher in cost than general electric parts. Thus, in order for the optical subscriber transmission system to become generally accepted, it is essential that the LED's be inexpensive.
A low yield in the manufacture of the LED's constitutes one of the major bars to the realization of the low-cost LED's. At present, during the manufacture of LED's, the respective layers are formed by a LPE (liquid phase epitaxy) method. However, the LPE method must be applied generally to small wafer areas, and a poor uniformity in the thickness of the respective films and in the composition is involved. Thus, it may be said that the LPE method is not suitable for a large quantity production of LED's. The use of the LPE method causes drooping at a layer-to-layer interface due to the lack of sharpness at the interface, resulting in the lowered light emission efficiency of the active layer. The use of the LPE method assures no excellent light emission characteristic. In order to increase input power to the optical fiber a lens is attached to the LED wafer. This step also presents a bar to the obtainment of low-cost LED's.
In the LD, on the other hand, the use of a MOCVD (Metal Organic Chemical Vapor Deposition) method in the crystal growth step is now under study. When the MOCVD method is used for the manufacture of LED's, the light emission efficiency is markedly lowered. In the case of the LD, since light is emitted due to the induced emission, the lowered light emission efficiency at the active layer offers no great problem. For the LED, on the other hand, the lowered light emission efficiency produces a fatal defect. As appreciated from the above, the use of the MOCVD method has been believed unsuitable for the manufacture of LED's.